Halo 3 skulls missing

So I had all the skulls and now one is missing the one where if you shoot grunts they turn into confetti and I re did the lvl and it is not there at all unless it's in the ground or something I can't find it any answerers?

The "Grunt Birthday Party" skull can be found in the "Crow's Nest" level, in the access tunnels with the Buggers flying around, just after a Cortana vision.

To find the skull, you need to (carefully) drop down to the level below, by moving backwards immediately after you fall off the ledge above. This can be easily missed, so take care.

Note: This is a "Silver" Skull, so when you pick it up, you will not get any notification that the skull has been collected, other than it appearing in the menu.